The present invention relates to photovoltaic infrared sensor systems in general, and in particular to such sensor systems employing mercury cadmium telluride semiconductor devices.
Mercury cadmium telluride is an intrinsic photodetector material used in sensor systems on military satellites employed for detection, designation, and discrimination of various targets via their infrared signatures. Such satellite electronic equipment must operate in an environment of natural space radiation and may also have a requirement to survive and perform after exposure to an atmospheric nuclear detonation. Effects of such detonations on spacebased electronic equipment include reduced long-term life due to absorption of high doses of ionizing radiation, noise in optical sensors, and changed component operating characteristics.
With respect to the mercury cadmium telluride semiconductor devices, the radiation causes type conversion on the surface of the tri-metal homojunction infrared sensors creating cross talk between adjacent same type doping regions. With cumulative radiation, the semiconductor device degrades in performance until it no longer functions, thereby rendering the sensor system ineffective in performing its mission.
Prior work in this area includes U.S. Pat. No. 4,063,095 which discloses dual sensing arrays with the second array shielded from radiation and used as a standard to compare against the output of the array that is irradiated. The second array never replaces the first array. Also, U.S. Pat. No. 4,217,490 discloses the use of three HgCdTe elements for use in an infrared detector, with each of the elements used for detecting a different band of the infrared spectrum. While each of these patents is suitable for its intended purpose, neither patent solves the problem of extending the useful life of the sensor system in a radiation environment.